Are they competing frameworks?
No, these are not competing and they are very different ideas.
RX is a library that is frequently used in applications that require real-time processing over a stream of events. RX gives you a library of functions to perform this with ease.
Message Queue (MQ) is more of an architecture component than a library. MQs are used to decouple event-producing logic and event-consuming logic. Example: on a user sign up page, you may want to perform a series of actions; one of it is sending of a welcome email to user. You may decide that you do not want the user to wait too long on the loading screen, therefore you use an MQ to simply trigger a “send welcome email” event to be processed later.
Can they be used in conjuction?
Yes. For example, in a file processing application -
1) Use RX to process a stream of CSV records
2) You can combine records, transform, etc
3) Take the final result and put it into a queue, then move on to next batch of CSV records
4) Meanwhile, a queue consumer reads that result above and performs an upload of the result to a remote file server